1. Field of the Invention
This invention relates to a novel construction for a membrane assembly suitable for solid-liquid separation, especially solid-liquid separation techniques designed to obtain purified water.
2. Description of the Related Art
Sand filtration and gravity precipitation techniques are known for performing solid-liquid separation in water purification treatments, treatments of sewage and waste water, and treatments of industrial waste water. However, the solid-liquid separation of such to-be-treated water (also referred to herein as object water) based on these conventional techniques often produces treated water of insufficient quality. To raise the quality to an acceptable level, vast and complex sites are needed to carry out the solid-liquid separation with these conventional techniques.
In recent years, the use of membrane modules containing separating membranes, such as precision filtration membranes or ultra-filtration membranes, have been proposed as alternative methods for solid-liquid separation of object water. When object water is subjected to filtration treatment with separation membranes, high quality treated water can be obtained, thus overcoming the drawbacks of the conventional sand filtration and gravity precipitation techniques.
However, prolonged use of separating membranes for solid-liquid separation of object water can cause suspended solids to clog the membranes, thereby decreasing the filtration flow rate and/or increasing the differential pressure between membranes. In order to restore a normal solid-liquid separation condition, the suspended solids are removed from the surfaces of the membranes by vibrating the separating membranes with diffusing air supplied from air diffusion pipes provided below the membrane modules. Vibration of the separating membranes only provides a temporary solution, however, since suspended solids tend to clog even diffusion-vibrated membranes, especially after long periods of use. Thus, it is necessary to conduct frequent maintenance work to maintain a normal filtration flow rate.
Japanese Patent Laid-Open No. 257378/1996 proposes a method in which a substantially parallel gas-liquid mixed flow is created in a membrane module unit by providing enclosure walls in direct contact with outermost membrane modules. However, although a parallel flow is created by this arrangement, the present inventors have found that this arrangement results in a low membrane scrubbing efficiency.
A system for moving air diffusion members has been proposed by Japanese Patent Laid-Open No. 24596/1996. In this system, movement of the diffusion members creates a turbulent flow of bubbles. This turbulent flow increases the efficiency of the system, but at the same time increases the power requirements for the system due to the need to move the diffusion members. Moreover, the movable members are prone to breakage, thereby increasing maintenance work frequency and cost.